Reply to: Increase of P-wave velocity due to melt in the mantle at the Gakkel Ridge

Yu and Singh argued that our seismic tomography model for the area of the Gakkel Ridge is not trustworthy and does not provide solid evidence for the proposed scenario of explosive volcanism development. They doubt the adequacy of the initial data analysis and express concerns that the SP phases might be associated with wave conversions in a sediment layer. In our response, we present several arguments on why it is highly unlikely. Yu and Singh also presented the Wadati diagram with a very low value of the Vp/Vs ratio. We have

It is well known that in an oceanic environment, phase conversions can occur at sharp structural boundaries not only at the seafloor but also below the seafloor. For example, an OBS refraction study close to a spreading center where a thick sedimentary layer is missing (Lata and Dunn, 2020, Marine Geology) indicate that P-to-S converted seismic phases arise from two shallow interfaces, one at ~80 m depth, and the other at 500-650 m depth. They also demonstrate that these shallow layers hold low S-wave velocity (< 1.0 km/s) and high Vp/Vs ration of over ~5. This means that such a thin layer can produce a remarkable delay of S-wave arrivals as in the study of Dr. Koulakov et al., and thus its effects need to be dealt with adequately in tomography analyses. This is the primary reasons why such tomography studies include "station corrections" for S wave arrivals. However, as far as I understand, Dr. Koulakov et al. has not taken it into account in their calculation.
I also agree with another comment by the criticizing authors that the picked sP waves are actually PsP-waves. However, Dr. Koulakov has not provided any convincing arguments on this.
Based on the considerations above, it seems to me that the authors haven't addressed the main concerns by the criticizing authors.

Reviewer #2 (Remarks to the Author):
This is an important discussion to be published.
(1) whether you judge that the criticism raised in the comment is valid, (2) whether it is likely to be of significant interest to the readers of the original Article. I believe that it is worth to publish the discussion about the point that there may be misinterpretation of the most important data in the original paper. In addition, the authors of the original paper should disclose the seismic waveform data they used.

Reviewer #1 (Remarks to the Author):
This is a reply by Dr. Koulakov et al. to a criticizing comment by Dr. Yu and Dr Singh.
As I pointed out in my review comments to the MA manuscript by Dr. Yu and Dr. Singh, the most important discussion here is whether the S-wave velocity model proposed by Dr. Koulakov et al. is accurate or not. This discussion is primarily dependent on the author's assumption that S-P converted phases they used for the tomography are S-waves converted to P-waves on the sea bottom. To rule out other possibilities of conversion patterns and locations, Koulakov et al. argue in their reply that "there is no evidence for a thick sedimentary layer in the study area. On the contrary, the refraction seismic surveys revealed normal oceanic near-surface P-wave velocities (2.4-3.9 km/s)6. In addition, gravity coring in the area of the 85°E volcano did not find any trace of soft sediments7". After reading the reply, however, I am still concerned that the Vs model has not been properly constructed since the significant delay of S waves is possible even in the absence of sedimentary layers. REP: We have considerably rewritten the manuscript by expanding the arguments on the validity of data analysis and placing them in the first place in the text.
The main reason why these phases cannot be associated with wave conversions in a sediment layer is that in this case the observed differential S-P times would merely depend on the properties of sediments where the station is located and would not be dependent on hypocentral distance. In fact, we clearly see that rays with later P-wave times corresponding to larger hypocentral distances always have larger differential times Ts-Tp. (L32-36) Another argument that the PS phases were identified correctly was the stable tomographic inversion and reasonable values of the obtained velocities and Vp/Vs ratio. If we misinterpreted our data and used them in our tomography inversion, we would never obtain such a good data fit for the S-wave residuals (0.06 s) and such a strong variance reduction (51%) as observed after the inversion in our case. Erroneous data would behave as outliers and would never provide any stable solution. (L37-42) It is not easy to debate about potential layers of sediments in the area considered in the paper in the absence of bottom seismometers and any special studies oriented to such types of shallow structures. Therefore, we have added information about a neighboring volcano zone on Gakkel Ridge having similar geological structure (L43-56). The available bottom seismometer data clearly show very strong S-phases, which are expected to be converted to the clear SP wave in the water layer. On the other hand, these records did not recognize any prominent Ps phase converted in shallow sediments. We can expect that a similar wave configuration exists in the area of our study.
It is well known that in an oceanic environment, phase conversions can occur at sharp structural boundaries not only at the seafloor but also below the seafloor. For example, an OBS refraction study close to a spreading center where a thick sedimentary layer is missing (Lata and Dunn, 2020, Marine Geology) indicate that P-to-S converted seismic phases arise from two shallow interfaces, one at ~80 m depth, and the other at 500-650 m depth. They also demonstrate that these shallow layers hold low S-wave velocity (< 1.0 km/s) and high Vp/Vs ration of over ~5. This means that such a thin layer can produce a remarkable delay of S-wave arrivals as in the study of Dr. Koulakov et al., and thus its effects need to be dealt with adequately in tomography analyses. This is the primary reasons why such tomography studies include "station corrections" for S wave arrivals. However, as far as I understand, Dr. Koulakov et al. has not taken it into account in their calculation.
REP: Regarding the sediment-converted phases, see our previous response. As for the station corrections, it was not possible to apply them in this study, because every station point while changing locations was considered as another station. It would create too many unknowns and make the inversion unstable.
I also agree with another comment by the criticizing authors that the picked sP waves are actually PsP-waves. However, Dr. Koulakov has not provided any convincing arguments on this.
Based on the considerations above, it seems to me that the authors haven't addressed the main concerns by the criticizing authors.
REP: In the new version of the manuscript, we have provided several additional arguments proving that the picked SP phases are real and not misinterpreted with PsP phases generated by conversions in the sediment layer (see our comments above).

Reviewer #2 (Remarks to the Author):
This is an important discussion to be published. REP: We do not understand this comment. In our opinion it is not easy to predict in this setting the amplitudes of a SP converted phase given that the seismometers are installed on a drifting ice floe. We base our phase identification on the knowledge that S phases in ocean bottom records in these settings are very strong and likely powerful enough to produce SP converted energy at the sea surface while converted phases from shallow interfaces have very small amplitudes in comparison to the S phase.
However, the data presented by Koulakov et al are only the waveform of one earthquake, and I do not have access to other data. I would need to check other data as well to judge the details. I am actually surprised the original paper does not present any other waveforms in the supplement. REP: In our previous version of the response, we have demonstrated that Yu and Singh used a wrong way of calculation of the Wadati diagram and therefore they obtained unrealistic values of Vp/Vs ratio. We have provided the calculations for both observed and synthetic data and estimated that the Vp/Vs ratio is equal to 1.77, which is a reasonable value for these settings.
(2) whether it is likely to be of significant interest to the readers of the original Article. I believe that it is worth to publish the discussion about the point that there may be misinterpretation of the most important data in the original paper. In addition, the authors of the original paper should disclose the seismic waveform data they used.
REP: Based on several arguments presented in the response file, we claim that any misinterpretation of our initial data is very unlikely. As we said earlier, the waveforms are now available in open access.
(3), could it be stated more concisely? Yu et al. concisely describes the claim.